I guess, this is for historical reasons. Mind that there is no such thing as a single, cached image. There's the downloaded content, a decoded bitmap derived from this, a buffer for any instance of the image, which may be clipped or distorted (and may have local color management applied, e.g., converted to 8-bit color range). (At least, it used to be that way. I faintly remember that this used to be a 4-step process.) When memory wasn't ample, any of these, but the instance buffer(s), may have been purged, and an instance buffer doesn't represent the original image anymore. So it makes sense to get a new clean image in the original encoding.

> They already have the image, why redownload it?

They don’t already have the image. They have part of the image. Because the connection hasn’t closed, as far as the browser is concerned, it’s still in the process of downloading it.